1. Field of Invention
The invention relates to an illumination system for illuminating a display device, which illumination system is provided with a light-emitting panel comprising
a front wall and, opposite thereto, a rear wall as well as a first and a second light-transmitting edge surface which are situated between the front wall and the rear wall,
at least one first light source being associated with the first edge surface and at least one second light source being associated with the second edge surface, and
in operation, light originating from the first light source being incident on the first light-transmitting edge surface, and light originating from the second light source being incident on the second light-transmitting edge surface, said light spreading in the light-emitting panel.
The invention also relates to a display device provided with said illumination system.
2. Description of Related Art
Such illumination systems are known per se and are also referred to as edge lighting systems. They are applied, inter alia, as backlighting of (image) display devices, for example for television receivers and monitors. Such illumination systems can particularly suitably be used as backlights for non-emissive displays, such as liquid crystal display devices, also referred to as LCD panels, which are employed in (portable) computers or (cordless) telephones.
Such display devices generally comprise a substrate provided with a regular pattern of picture elements (pixels), which are each controlled by at least one electrode. In order to form an image or a datagraphic display in a relevant area of a (display) screen of the (image) display device, the display device employs a control circuit. In a LCD device, the light originating from the backlight is modulated by means of a switch or a modulator, whereby various types of liquid crystal effects are employed. In addition, the display may be based on electrophoretic or electromechanical effects.
In the illumination systems mentioned in the opening paragraph, a tubular low-pressure mercury-vapor discharge lamp, for example one or more compact fluorescent lamps, is generally used as the light source, the light emitted, in operation, by the light source being coupled into the light-emitting panel which serves as an optical waveguide. This optical waveguide generally forms a comparatively thin, flat panel which is made of, for example, a synthetic resin or glass, with light being transported through the optical waveguide under the influence of (total) internal reflection.
Such an illumination system may also be provided with an alternative light source in the form of a plurality of opto-electronic elements, also referred to as electro-optic elements, such as electroluminescent elements, in particular light-emitting diodes (LEDs). These light sources are generally provided so as to be close to or abut against a light-transmitting edge surface of the light-emitting panel, so that, in operation, light originating from the light source is incident on the light-transmitting edge surface and spreads into the panel.
In U.S. Pat. No. 5,664,862, a description is given of an illumination system for illuminating a LCD panel. For the light source use is made of incandescent lamps. Other light sources, such as fluorescent lamps and light-emitting diodes are also mentioned. In operation, the light source couples light into the light-emitting panel via a first light-transmitting edge surface, said light-emitting panel also being referred to as light guide. The rear wall of the light-emitting panel is provided with coupling-out members, also referred to as facets. These facets are preferably shaped such that light entering the panel via the first light-transmitting edge surface is reflected, predominantly in the longitudinal direction of the panel, in the direction of the opposite second light-transmitting edge surface. The light reflected at this second light-transmitting edge surface strikes the facets, as a result of which the light is partly deflected so as to be coupled out of the light-emitting panel, and partly continues on its way through the light-emitting panel. The facets are provided in the rear wall in such a manner that a uniform light distribution on the LCD panel to be illuminated is obtained.
An illumination system of the above-mentioned type has the disadvantage that the light distribution in the light-emitting panel, particularly close to the light source, is insufficiently uniform. As a result, the display device is illuminated insufficiently uniformly.
It is an object of the invention to completely or partly obviate said drawbacks. The invention more particularly aims at providing an illumination system of the type mentioned in the opening paragraph, wherein the uniformity of the light distribution of the illumination system, and hence the uniformity with which the display device is illuminated, is improved.
In accordance with the invention, this object is achieved in that,
a first sub-surface of the front wall or of the rear wall of the light-emitting panel is provided with first coupling-out members for coupling light out of the panel, which light predominantly originates from the first light source, and
in that a second sub-surface of the front wall or of the rear wall of the light-emitting panel is provided with second coupling-out members for coupling light out of the panel, which light predominantly originates from the second light source,
the concentration of the first coupling-out members near the second edge surface being higher than the concentration of the second coupling-out members, and
the concentration of the second coupling-out members near the first edge surface being higher than the concentration of the first coupling-out members.
By dividing the coupling-out members into first coupling-out members, which predominantly couple out light originating from the first light source, and second coupling-out members, which predominantly couple out light originating from the second light source, and by clustering these coupling-out members into, respectively, a first sub-surface and a second sub-surface, said sub-surfaces being positioned at some distance from the edge surface where the light of the relevant first or second light source is coupled into the light-emitting panel, it becomes possible to use a part of the light-emitting panel to mix light originating from the respective light sources before the light of these light sources contacts coupling-out members which couple light out of the light-emitting panel. By raising the concentration of the first coupling-out members close to the second edge surface, which first coupling-out members have a preference for coupling-out light originating (directly) from the first light source, it is achieved that light originating from the first light source first travels through the light-emitting panel for a while, as a result of which the risk of the light striking coupling-out members coupling out said light is comparatively small. In other words, if the concentration of the second coupling-out members near the first edge surface is chosen to be higher than the concentration of the first coupling-out members, a substantial part of the light originating (directly) from the first light source is coupled out of the light-emitting panel at locations which are situated at a comparatively large distance from the first edge surface. The same is true for light originating from the second light source. By raising the concentration of the second coupling-out members close to the first edge surface, which second coupling-out members have a preference for coupling out light originating (directly) from the second light source, it is achieved that light originating from the second light source first travels through the light-emitting panel for a while, as a result of which the risk of the light striking coupling-out members coupling out said light is comparatively small.
Using a part of the light-emitting panel to enhance the mixing of light has the additional advantage that a separate light-mixing chamber wherein light originating from the light source(s) is mixed before being coupled into the light-emitting panel is no longer necessary, which leads to a considerable saving in space in the illumination system. As a result of the measure in accordance with the invention, a part of the light-emitting panel is used, as it were, as a type of xe2x80x9cinternalxe2x80x9d light-mixing chamber.
Mixing light in the light-emitting panel is very advantageous if the light source comprises a collection of light sources of different colors, for example blue, green, (amber) and red light sources, for example a collection of light-emitting diodes of different colors. Such a collection can be provided, in the form of an array, so as to lie against the light-transmitting edge surface of the light-emitting panel, and the colors, the wavelength and the mutual intensity of the various light sources of different colors can be selected to be such that, after sufficient mixing, white light of a certain (adjustable) color temperature is formed. Color effects occurring at the edges of the light-emitting panel and being visible in the display device as a result of insufficient mixing of light are undesirable.
By virtue of the measure in accordance with the invention, a kind of (optical) division is brought about between the coupling-out members which couple out light originating from the first light source, and the coupling-out members which couple out light originating from the second light source.
In general, in the illumination system in accordance with the invention, the respective sub-surfaces co-operate to uniformly illuminate the display device. The choice of the sub-surfaces and the distribution of the coupling-out members over said sub-surfaces is such that light which, in operation, is emitted by each of the sub-surfaces uniformly illuminates the display device.
By virtue of the measure in accordance with the invention, the uniformity of the distribution of the light emitted by the illumination system is improved. As a result, a more uniform illumination of the (image) display device is obtained.
In the known illumination system, the coupling-out members are formed such that light entering the panel via the light-transmitting edge surface and striking the coupling-out members is reflected substantially in the longitudinal direction of the panel, i.e. in the direction of the opposite edge surface. Light originating from the light-transmitting edge surface which, upon reflection from this opposite edge surface, strikes the coupling-out members can be subsequently coupled out of the panel in that the shape of the coupling-out members is different for this propagation direction. If light cannot be coupled out until after said reflection from the edge surface opposite the light-transmitting edge surface, then the efficiency of the light-emitting panel is adversely affected.
The shape of the coupling-out members influences the way in which light issues from the light-emitting panel. A favorable embodiment of the illumination system is characterized in accordance with the invention in that the first and second coupling-out members are wedge-shaped,
a side face of the first coupling-out members facing the second light source extending at least substantially parallel to the second edge surface, and
a side face of the second coupling-out members facing the first light source extending at least substantially parallel to the first edge surface.
As the side face of the first coupling-out members facing the second light source extends transversely to the (principal) direction in which the second light source emits light, light is not coupled out of the light-emitting panel when light emitted (directly) by the second light source strikes the first coupling-out members. As the side face of the second coupling-out members facing the first light source extends transversely to the (principal) direction in which the first light source emits light, light is not coupled out of the light-emitting panel when light emitted (directly) by the first light source strikes the second coupling-out members. Light reflected in this manner moves substantially parallel to the longitudinal direction of the panel. Light can only be coupled out of the light-emitting panel if it strikes a side face of a coupling-out member which does not extend transversely to the (principal) direction of propagation of the light. By reducing the possibility that light originating directly from one of the light sources proximate to the edge surface with which the relevant light source is associated is coupled out, the mixing of light proximate to the relevant light source is improved.
The shape of the sub-surfaces and their position relative to each other, in conjunction with the way in which the light coupling-out members are distributed over the sub-surfaces, influence the way in which light issues from the light-emitting panel.
A favorable embodiment of the illumination system is characterized in accordance with the invention in that each of the sub-surfaces comprises a single surface which covers approximately half the front wall or rear wall of the light-emitting panel,
the coupling-out members being provided such that the degree to which light is coupled out by the sub-surfaces is at least substantially constant.
In this preferred embodiment of the illumination system, the first coupling-out members are mainly responsible for the illumination of one half of the display device, and the second coupling-out members are mainly responsible for the illumination of the other half of the display device. Preferably, the two sub-surfaces demonstrate some overlap in order to improve the uniformity of the illumination of the display device. The distribution of the coupling-out members over said sub-surfaces is so uniform that the light emitted by the sub-surfaces very uniformly illuminates the corresponding part of the display device. A uniformly illuminated display device is obtained by providing the sub-surfaces, preferably, on a part of the front wall or the rear wall which is situated at a (maximum) distance from the light-transmitting edge surface of the light source.
It is particularly favorable if the first sub-surface of the light-emitting panel illuminates predominantly one half of the display device, for example the lower half, while the second sub-surface of the light-emitting panel illuminates predominantly the other half of the display device, for example the upper half. This has the advantage that the illumination levels of the two halves of the display device may be different. This has the further advantage that the display screen of the display device is suitable for spatial sequential operation. In an alternative embodiment, a ridge is provided in one of the sub-surfaces or in both sub-surfaces, as a result of which mixing of light between the two halves of the display device is effectively precluded.
Apart from the above-described uniform distribution of the coupling-out members over the sub-surfaces, it may be advantageous, in accordance with alternative embodiments, to distribute the coupling-out members non-uniformly over the sub-surfaces. For this purpose, a suitable embodiment of the illumination system is characterized in that the sub-surfaces overlap each other partly.
Also a suitably chosen inhomogeneous distribution of the coupling-out members allows the light emitted by the various sub-surfaces to illuminate the display device very uniformly. For this purpose, a preferred, alternative embodiment of the illumination system in accordance with the invention is characterized in that the sub-surfaces extend over a substantial part of the front wall or the rear wall of the light-emitting panel, the coupling-out members being provided such that the amount of light coupled out changes as a function of the distance to the light-transmitting edge surface.
Preferably, the coupling-out members are provided throughout the front wall or rear wall of the light-emitting panel, the amount of light coupled out being comparatively small close to the light-transmitting edge surfaces and comparatively large close to the opposite edge surfaces.
In accordance with a particularly advantageous distribution, the coupling-out members are arranged such that the amount of light coupled out changes linearly as a function of the distance to the light-transmitting edge surface. In an alternative, favorable embodiment, the coupling-out members are arranged such that the coupling-out of light changes as the square of a sine or cosine.
The coupling-out members are preferably arranged on the rear wall of the light-emitting panel.
Preferably, the light source comprises at least a light-emitting diode (LED) having a luminous flux of at least 5 lm. LEDs having such a high output are also referred to as LED power packages. The use of these high-efficiency, high-output LEDs has the specific advantage that the number of LEDs necessary for a desired, comparatively high luminous output can be relatively small. This has a favorable effect on the compactness and the efficiency of the illumination system to be manufactured. Further advantages of the use of LEDs are a comparatively very long service life, comparatively low energy costs and comparatively low maintenance costs of an illumination system with LEDs.
The application of LEDs has the further advantage that dynamic illumination possibilities are obtained. For this purpose, an embodiment of the illumination system is characterized in that the light-emitting panel is provided with a sensor for measuring the optical properties of the light which, in operation, is emitted by the light source(s). If different types of LEDs are combined and/or LEDs of different colors are employed, colors can be suitably mixed, for example, to obtain white light of the desired color temperature. In addition, color changes can be obtained independent of the state of the display device. The sensor can be provided, for example, on an edge surface of the light-emitting panel.
In a further preferred embodiment, the illumination system comprises control electronics for changing the luminous flux of the light source. Suitable control electronics enables the desired illumination effects to be achieved and the uniformity of the coupled-out light to be improved. In addition, a suitable combination of LEDs enables white light to be obtained, a desired color temperature of which can be adjusted by means of control electronics.
In a preferred embodiment of the illumination system, the light source comprises at least two light-emitting diodes having different light emission wavelengths. Such diodes emit light in a predetermined wavelength range. The LEDs comprise, preferably, the well-known combinations of blue, green and red LEDs or combinations of red, green, blue and amber LEDs.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.